Initial journald log provider attempt

**What**
- journald log provider using exec to journalctl
```
journalctl -t <namespace>:<name>  --output=json --since=<timestamp> <--follow> --output-fields=SYSLOG_IDENTIFIER,MESSAGE,_PID,_SOURCE_REALTIME_TIMESTAMP
```
- This can be tested manually using `faas-cli logs` as normal, e.g.
  `faas-cli logs nodeinfo` should tail the last 5 mins of logs.
- Very basic tests ensuring that the `journalctl` comamand is correctly
  construction and that the json log entrys are parsed correctly.
- Add simple e2e test to grep the function logs

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

pkg/provider/handlers/deploy.go

@@ -18,6 +18,7 @@ import (
 	"github.com/docker/distribution/reference"
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/openfaas/faas-provider/types"
+	faasd "github.com/openfaas/faasd/pkg"
 	cninetwork "github.com/openfaas/faasd/pkg/cninetwork"
 	"github.com/openfaas/faasd/pkg/service"
 	"github.com/pkg/errors"
@@ -52,7 +53,7 @@ func MakeDeployHandler(client *containerd.Client, cni gocni.CNI, secretMountPath
 		}
 
 		name := req.Service
-		ctx := namespaces.WithNamespace(context.Background(), FunctionNamespace)
+		ctx := namespaces.WithNamespace(context.Background(), faasd.FunctionNamespace)
 
 		deployErr := deploy(ctx, req, client, cni, secretMountPath, alwaysPull)
 		if deployErr != nil {